L. N. Bulaevskiǐ

Abstract Superconductivity and ferromagnetic ordering are two antagonistic types of ordering, and their mutual influence leads to many interesting phenomena which have been studied recently in ternary compounds. Theoretical analysis of ferromagnetic materials which are type II superconductors near the superconducting transition point T cl shows that they become type I near the magnetic transition point T M. The proposed theory constructed for the case T M « T cl predicts the formation of a transverse domain-like (DS phase) magnetic structure below T M. The electronic spectrum appears to be gapless in the DS phase of clean compounds with a re-entrant transition. The change from type II to type I behaviour as the sample is cooled to T M has been observed in ErRh4B4. Experimental data for HoMo6S8, HoMo6Se8 and ErRh4B4 give evidence for the coexistence of super-conductivity and non-uniform magnetic ordering below T M. Mutual influence of superconducting and magnetic orderings is also studied.

Strongly anisotropic layered superconductors are considered within the Lawrence-Doniach model. The differential finite-difference sine-Gordon equation is derived for the order-parameter phase differences; boundary conditions are formulated for a vortex lattice. The structure of a tilted vortex is considered. The line energy of a single tilted vortex and the free energy of a tilted vortex lattice in moderate magnetic fields are calculated. Deviations from the three-dimensional London theory are substantial for field orientations close to the layers. As the applied field approaches the ab plane, the orientational lock-in transition (tilted-parallel lattice) occurs provided ${\ensuremath{\lambda}}_{\mathit{J}}$\ensuremath{\lesssim}${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$; here ${\ensuremath{\lambda}}_{\mathit{J}}$=\ensuremath{\gamma}s is the Josephson length, \ensuremath{\gamma} is the anisotropy parameter, and s is the interlayer spacing. If ${\ensuremath{\lambda}}_{\mathit{J}}$\ensuremath{\gtrsim}${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$, the tilted lattice transforms first into a new type of vortex arrangement that consists of sets of coexisting parallel and perpendicular vortices (combined lattice). Then, as the field further approaches the ab plane, the combined lattice goes over to the parallel one. The angular dependence of the torque is evaluated for tilted, combined, and parallel lattices, which allows one to experimentally distinguish these phases.

The temperature ${\mathit{T}}_{\mathit{s}}$ of spontaneous creation of vortex lines in Josephson coupled layered superconductors is obtained by taking into account the entropy contribution to the free energy due to thermal distortions. For Bi- and Ti-based high-${\mathit{T}}_{\mathit{c}}$ superconductors and superlattices, the superconducting critical temperature ${\mathit{T}}_{\mathit{s}}$ lies noticeably below the mean-field transition temperature ${\mathit{T}}_{\mathit{c}0}$(${\mathit{T}}_{\mathit{c}0}$-${\mathit{T}}_{\mathit{s}}$\ensuremath{\approxeq}4 K or more). The contribution of thermal distortions to the free enery causes significant changes in the temperature and field dependences of magnetization below ${\mathit{T}}_{\mathit{s}}$ seen in Bi compounds.

A grain-structure model is used to obtain the low-temperature transport properties of polycrystalline Bi-based high-temperature superconducting tapes, including the magnetic-field-dependent critical current. The grain structure is regarded as resembling a brick wall, such that the net horizontal supercurrent passes from brick to brick chiefly through the horizontal junctions between bricks. A high-field critical-current plateau is predicted, assuming inhomogeneous Josephson junctions between highly anisotropic superconducting grains.